This invention relates to the field of nonvolatile memories. In particulars this invention is drawn to multiple bit magnetic memory cells.
One type of nonvolatile memory relies on magnetoresistive principles. A particularly useful magnetoresistive effect is referred to as the giant magnetoresistive effect. In one embodiment, GMR-based magnetic memory cells are multilayered structured comprising conductive magnetic layers and a conductive non-magnetic metallic layer. The magnetic state of the cell is determined by the relative orientation of a magnetic vector in one magnetic layer to a magnetic vector in another magnetic layer (e.g., parallel or anti-parallel). The resistance of the cell differs according to the relative orientations of the magnetic vectors. Accordingly the state of the cell can be determined by applying a voltage across the cell and measuring a resulting sense current.
One type of GMR memory cell xe2x80x9cpinsxe2x80x9d the magnetic vector of one of the layers. The layer containing the pinned magnetic vector is referred to as the reference layer. The state of the cell is then controlled by varying the orientation of the magnetic vector in the other magnetic layer. The non-pinned magnetic layer is also referred to as the data layer. The orientation of the magnetic vector in the data layer is controlled through the application of a magnetic field that has little effect on the magnetic vector of the pinned layer at least at low field intensities. This type of cell is referred to as a spin valve cell or as a GMR cell.
A spin dependent tunneling cell uses a non-conductive barrier layer such as dielectric material instead of a metallic layer between the reference and data layers. The transport mechanism between reference and data layers is tunneling through the barrier layer. Thus the cell may be referred to as a tunneling magnetoresistive (TMR) cell. The TMR cell offers a number of advantages over the conductive GMR cells. In particular, a greater change in resistance is observed with the TMR cell structure as opposed to the GMR cell structure.
With respect to both GMR and TMR cells structures, the reference or pinned layer can create a magnetostatic or demagnetization field in the data layer due to the proximity of the reference and data layers. The magnetostatic field from the reference layer can become the dominant field and thus permanently change the magnetization of the data layer. Data storage thus becomes unreliable. Although the height of the metallic separation layer can be increased to compensate for the magnetostatic field in the GMR cell structures, the height of the barrier in TMR structures is constrained by the necessity of having the barrier thin enough to accommodate quantum tunneling.
One prior art magnetic memory cell structure utilizes two data layers to store multiple bits. One disadvantage of this structure is that the length-to-width aspect ratio of each layer needs to be 5 or more to reduce the effect of the magnetostatic field thus reducing the storage density of a device constructed from such cells. Another disadvantage is that the prior art structure is capable of realizing only two resistance values. Determining the state of the device requires a destructive readback scheme followed by a rewrite operation.
In view of limitations of known systems and methods a magnetic memory cell structure and method of fabricating the structure are described.
One embodiment of a multibit magnetic memory cell includes first and second data layers. An antiferromagnetically coupled pair of reference layers is disposed between the first and second data layers. In one embodiment the first and second data layers have distinct coercivities. The memory cell structure comprises a separation layer separating each of the first and second data layers from the antiferromagnetically coupled layer pair. In one embodiment, the separation layers are nonconductive. In an alternative embodiment, the separation layers are conductive.
In various embodiments, heights and lengths of the layers may be selected to ameliorate magnetostatic fields between the data layers. In one embodiment, a height of the first and second data layers is not the same. In another embodiment, a height of the separation layer between the second data layer and the antiferromagnetically coupled pair of layers is not the same as a height of the separation layer between the first data layer and the antiferromagnetically coupled pair of reference layers.
A method of fabricating a multibit magnetic memory cell includes the step of forming a first ferromagnetic layer over a semiconductor substrate. The method includes the step of forming a second ferromagnetic layer over the first ferromagnetic layer. An antiferromagnetically coupled layer pair is formed such that the antiferromagnetically coupled layer pair is disposed between the first and second ferromagnetic layers.